a 7.0 PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL CONDITIONING 1 Introduction 2 Bridge Circuits 3 Amplifiers for Signal Conditioning 4 Strain, Force, Pressure, and Flow Measurements 5 High Impedance Sensors 6 Position and Motion Sensors n 7 Temperature Sensors 8 ADCs for Signal Conditioning 9 Smart Sensors 10 Hardware Design Techniques
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PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL …PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL CONDITIONING 1 Introduction 2 Bridge Circuits 3 Amplifiers for Signal Conditioning 4
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a 7.0
PRACTICAL DESIGN TECHNIQUES FORSENSOR SIGNAL CONDITIONING
1 Introduction
2 Bridge Circuits
3 Amplifiers for Signal Conditioning
4 Strain, Force, Pressure, and Flow Measurements
5 High Impedance Sensors
6 Position and Motion Sensors
n 7 Temperature Sensors8 ADCs for Signal Conditioning
9 Smart Sensors
10 Hardware Design Techniques
a 7.1
n Monitoringu Portable Equipmentu CPU Temperatureu Battery Temperatureu Ambient Temperature
n Compensationu Oscillator Drift in Cellular Phonesu Thermocouple Cold-Junction Compensation
n Controlu Battery Chargingu Process Control
APPLICATIONS OF TEMPERATURE SENSORS
a 7.2
TYPES OF TEMPERATURE SENSORS
THERMOCOUPLE RTD THERMISTOR SEMICONDUCTOR
Widest Range:
–184ºC to +2300ºC
Range:
–200ºC to +850ºC
Range:
0ºC to +100ºC
Range:
–55ºC to +150ºC
High Accuracy and
Repeatability
Fair Linearity Poor Linearity Linearity: 1ºC
Accuracy: 1ºC
Needs Cold Junction
Compensation
Requires
Excitation
Requires
Excitation
Requires Excitation
Low-Voltage Output Low Cost High Sensitivity 10mV/K, 20mV/K,
or 1µA/K TypicalOutput
a 7.3
COMMON THERMOCOUPLES
JUNCTION MATERIALS
TYPICAL
USEFUL
RANGE (ºC)
NOMINAL
SENSITIVITY
(µV/ºC)
ANSI
DESIGNATION
Platinum (6%)/ Rhodium-
Platinum (30%)/Rhodium
38 to 1800 7.7 B
Tungsten (5%)/Rhenium -
Tungsten (26%)/Rhenium
0 to 2300 16 C
Chromel - Constantan 0 to 982 76 E
Iron - Constantan 0 to 760 55 J
Chromel - Alumel –184 to 1260 39 K
Platinum (13%)/Rhodium-
Platinum
0 to 1593 11.7 R
Platinum (10%)/Rhodium-
Platinum
0 to 1538 10.4 S
Copper-Constantan –184 to 400 45 T
a 7.4
THERMOCOUPLE OUTPUT VOLTAGES FORTYPE J, K, AND S THERMOCOUPLES
-250 0 250 500 750 1000 1250 1500 1750-10
0
10
20
30
40
50
60
TH
ER
MO
CO
UP
LE
OU
TP
UT
VO
LT
AG
E (
mV
)
TEMPERATURE (°C)
TYPE J
TYPE K
TYPE S
a 7.5
THERMOCOUPLE SEEBECK COEFFICIENTVERSUS TEMPERATURE
-250 0 250 500 750 1000 1250 1500 17500
10
20
30
40
50
60
70
SE
EB
EC
K C
OE
FF
ICIE
NT
- µ
V/ °
C
TEMPERATURE (°C)
TYPE J
TYPE K
TYPE S
a 7.6
THERMOCOUPLE BASICS
T1
Metal A
Metal B
ThermoelectricEMF
RMetal A Metal A
R = Total Circuit ResistanceI = (V1 – V2) / R
V1 T1 V2T2
V1 – V2
Metal B
Metal A Metal A
V1
V1
T1
T1
T2
T2
V2
V2
VMetal AMetal A
Copper Copper
Metal BMetal B
T3 T4
V = V1 – V2, If T3 = T4
A. THERMOELECTRIC VOLTAGE
B. THERMOCOUPLE
C. THERMOCOUPLE MEASUREMENT
D. THERMOCOUPLE MEASUREMENT
I
V1
a 7.7
CLASSICAL COLD-JUNCTION COMPENSATION USING ANICE-POINT (0°C) REFERENCE JUNCTION
METAL A METAL A
METAL B
ICEBATH
0°C
V(0°C)
T1 V1
V1 – V(0°C)
T2
a 7.8
USING A TEMPERATURE SENSORFOR COLD-JUNCTION COMPENSATION
TEMPERATURECOMPENSATION
CIRCUIT
TEMPSENSOR
T2V(T2)T1 V(T1)
V(OUT)
V(COMP)
SAMETEMP
METAL A
METAL B
METAL A
COPPERCOPPER
ISOTHERMAL BLOCKV(COMP) = f(T2)
V(OUT) = V(T1) – V(T2) + V(COMP)
IF V(COMP) = V(T2) – V(0°C), THEN
V(OUT) = V(T1) – V(0°C)
a 7.9
TERMINATING THERMOCOUPLE LEADSDIRECTLY TO AN ISOTHERMAL BLOCK
TEMPERATURECOMPENSATION
CIRCUITTEMPSENSOR
METAL A
METAL B
COPPER
COPPER
COPPER
V(OUT) = V1 – V(0°C)
T1 V1
T2
T2
ISOTHERMAL BLOCK
a 7.10
R1*24.9kΩΩ
USING A TEMPERATURE SENSOR FORCOLD-JUNCTION COMPENSATION (TMP35)